Protonic conduction in rare earth orthophosphates with the monazite structure

Electrical conduction in rare earth orthophosphates, LnPO4 (Ln=La, Pr, Nd and Sm), with the monazite structure (P21/n) was investigated by using conductivity measurements at 500–925 °C. From the conductivities of undoped and 1 mol% Sr-doped LnPO4 under wet (H2O and D2O) and dry atmospheres, it was found that LnPO4 began to conduct protons under wet atmosphere by substituting Sr for Ln. The conductivity behavior of 1 mol% Sr-doped LnPO4 versus p(H2O) and p(O2) was discussed in terms of the defect equilibria. It was concluded that protonic conduction was dominant in the materials though electron holes contributed slightly to the total conductivity as temperature increased. All the 1 mol% Sr-doped LnPO4 investigated in this study exhibited similar electrical conduction regardless of rare earth element used.